Before describing the blasting practice in quarries we shall describe liquid oxygen explosives.
Liquid Oxygen Explosives
Oxygen gas liquefies at—183°C. A given volume of liquid oxygen, when gaseous, is equivalent to 840 times at N.T.P. i.e. it has as much oxygen as would be available from 4000 times its volume of atmospheric air. If a combustible ingredient, made in the form of a cartridge, is soaked in liquid oxygen and then subjected to reaction of combustion, the reaction takes place with such terrific speed that large volume of gas is instantaneously released at high temperature so as to cause explosion. The velocity of detonation under suitable conditions of confinement can be faster than 5000 m/sec. This is the principle behind the use of liquid oxygen (LOX) as explosive.
LOX is used for removal of overburden as well as mineral in the quarries of coal as well as metalliferous mines. But its use is prohibited in underground coal mines. LOX is marketed by Indian Oxygen Ltd. in cartridges of two types.
(i) Small cartridges of dia. 25 mm to 90 mm
(ii) Large cartridges of dia. over 100 mm upto 210 mm
Fig. 8.18. A Lox cartridge
For small as well as large cartridges there is a "standard" cartridge. For small group the standard cartridge is 38 mm dia. x 300 mm long. The standard cartridge of large group is called "Full cartridge" and it is 190 mm dia x 600 mm long. The other cartridges are specified in terms of volume of the standard cartridge. In blasting performance, a full cartridge of LOX according to LOX dealers and results in the field, is considered equivalent to 18 kg of conventional explosives.
For those areas where the demand of liquid oxygen is heavy, the Indian Oxygen Ltd. has established central depots equipped with storage tanks and other arrangements of preparing LOX cartridges to be supplied to consumers about, an hour or two before charging into blast holes. Such depots are at Bermo, Kathara, Lohardaga, and centrally located centres in mining areas. Liquid oxygen is transported from factories in special vessels on trucks.
A LOX cartridge ready for blasting is prepared at the depots by soaking an absorbent cartridge in liquid oxygen. The basic ingredient of an absorbent cartridge is a cellulosic substance like crushed jute-stalk or other agricultural product though other substances such as hydrocarbons or metallic powders are used to impart to the soaked cartridge the properties of an industrial explosive. The absorbent cartridge for use in coal contains a special composition called "Loxite-C", with a view to reduce the temperature-of the gaseous products after the blasting and such cartridges for use in coal are -called LOX-C. The absorbent cartridge consists of the above ingredients wrapped in paper covered by cloth. Loxite factory at Ranchi manufactures absorbent cartridges which are kept in stock at the depots. LOX cartridges are not stored in colliery magazines, but are supplied by depots on 2 to 3 hours' prior intimation of the exact requirement for blasting.
For small consumers within 300 km of oxygen factories liquid oxygen, is supplied in special containers of 26 litres by train. During transit some evaporation, 5 to 10%, does take place. The small consumers have to obtain loxite absorbent cartridges, soaking boxes and other equipment to prepare LOX cartridges on the spot. Only small cartridges are prepared at the quarries in this manner for use in holes drilled by jack hammers or wagon drills.
LOX cartridges are inflammable and the flow of gaseous oxygen emanating from a cartridge will cause smouldering material, glowing coals, and cigarette stubs to burst into flames. LOX should therefore be kept away from such burning or smouldering material.
Characteristics of LQX are not constant. It depends much on the time that has elapsed between removal from the soaking vessels and firing. It is therefore not possible to specify the weight strength of LOX cartridge. LOX does not require a booster for blasting.
The LOX cartridges should be used in the field without delay {within half an hour in the case of large cartridges) to prevent loss of absorbed liquid oxygen. Grease or oil should not come in contact with the cartridges at any stage. If the oxygen of a LOX cartridge evaporates, the absorbent cartridge can be used again to prepare LOX cartridge at the depot. The ‘Life’ of a LOX cartridge in open is about one hour i.e. its full blasting power is available when used within that period but diminishes gradually after that period. The "Life" however, considerably increases in the confinement of the hole. The cartridge loses its oxygen by gradual evaporation if it comes in contact with water. For use in watery holes only H-type cartridges wrapped in a polythene bag before lowering in the hole should be used. The toe will not be sufficiently loosened if LOX is used without such precautions in watery holes.
LOX can be fired with or without the help of a detonator. A hole charged with LOX can be fired with safety fuse alone like gunpowder. In small-hole blasting, detonators are not much used as safety fuse is economic. Holes deeper than 3 m are fired by a detonating fuse.
Blasting practice in opencast mines
Efficient blasting should give such rock fragmentation as to eliminate the need for secondary blasting. The fragmented rock should be easily handled by the bucket of the shovel. For mineral which has to go to crushers, the size should not be more than the, input size for primary crusher.
Quantity of explosive consumption
The volume of rock broken by explosive is given by the following formula
Rock blasted per hole (solid m3) — depth of hole x burden x spacing.
Rock blasted by a round of holes = depth x burden x spacing x number of holes in the round
The following table gives an idea of explosive consumption in quarries (O.CG. or 80% special gelatine) for various rocks under Indian conditions (Solid m3).
Fig. 8.19. S-Spacing; B-Burden
Rock
1. Bituminous coal
2. Medium hard shales, sandstones, gypsum
3. Sandy shales, sandstone
4. Massive sandstone, limestone, laterite
5. Very hard shales, marbles, dolomites, limestones, magnesites, hematite
6. Extremely hard limestone, conglomerate
7. Gneiss, granite, amphibolite, schist
Explosive consumption kg/m3
0.1
0.2
0.3
0.4
0.5
0.7
0.9-1
Depth and pattern of blast holes
The depth of vertical blast holes in coal is generally equal to the height of the bench. In hard rock, like sandstone, laterite, hematite, etc. the depth should be 0.5 to 1 m deeper below the level of bench floor. This loosens the toe. In any case, the hole should terminate in hard rock and not in the soft one; otherwise the force of explosive is wasted.
Burden is the minimum distance from the face to the blast hole (the berm usually refers to burden at the top of the face).
Toe is the projection of the bottom of a face beyond the crest. Sometimes the bottom edge of the bench is referred to as toe.
In hard rocks like laterite and hematite the spacing and burden vary from 0.3 to 0.4 times the height of bench. In less hard rocks like coal and sandstone, the spacing and burden vary from 0.5 to 0.8 times the height of bench. The exact dimensions depend upon the hole diameter, type of explosive used, type of the rock, nature of rock consolidation and the angle of cleats or laminations with the blast hole.
Preparation of primer cartridge and charging blast holes.
For blasting with detonating fuse the primer cartridge of a booster, O.C.G. and similar gelatinous or semi-gelatinous explosive is prepared by pricking a through hole in it, 10-15 cm below the top with a brass or aluminium pricker and threading detonating fuse through it. A knot is made at the end of the fuse in contact with the cartridge to prevent the former from coming out. In the case of LOX, to prepare a primer cartridge no hole is pierced in the cartridge but the detonating fuse is coiled at the neck of the cartridge in the form of a loose knot so that some length of the fuse is in intimate contact with the explosive along its length. Aquadyne and Superdyne slurry explosives which are in the form of cartridges closed at both ends by strong rubber bands can be made as primer cartridges by tying a detonating fuse round the cartridge. The short length of fuse at the knot end can be inserted into a small hole pierced in the cartridge by a pricker. The slurry is viscous enough not to flow out of the, small hole.
The primer cartridge is always placed at the bottom of the hole in case of blasting with O.C.G. and similar explosives and also in case of LOX, ANFO, and slurry explosives. It is generally lowered by the detonating fuse tied to it. LOX partridges have a loop of copper wire round the neck. A self detaching hook is attached to the "copper wire loop and the cartridge lowered in the hole. After the cartridge rests in its place in the hole the self detaching hook comes out of the copper wire loop when the rope is slack. Cartridges of other explosives including ANFO or slurry explosive are dropped or lowered into the hole. ANFO mixture or slurry which is not in the form of cartridges is poured into the hole. In the case of ANFO or slurry explosive a few cartridges of booster or O.C.G. in the middle of the total explosive column or at the top is essential for good blasting performance. The quantity of O.C.G. or other explosive as booster in the total explosive column if ANFO or slurry explosive is used is nearly 15-20%.
Deck loading i.e. separating the explosive charges into sections by placing a column of stemming between groups of cartridges is a useful technique for obtaining better rock fragmentation especially where soft and hard rock are encountered in alternate layers. A primer cartridge is placed at the deck charge also and the detonating fuse of bottom-most primer cartridge is threaded through the primer cartridge of the deck charge. (Fig. 8.20)
The drill cuttings in overburden are used for stemming after moistening them with water. The stemming is packed by tampers or a wooden tamping dolly.
LOX cartridges, if they have to be used in watery holes, should be enclosed in polythene bags before lowering. After LOX cartridges are lowered in the hole, sand bags are slipped on top of them to anchor them at the bottom of the hole and to prevent them from floating to the surface as liquid oxygen evaporates. The hole is then completely stemmed by overburden cuttings. Gas bubbling through the water should not distract attention as the blasting efficiency is not affected if blasting is performed within a reasonable time.
The detonating fuse is cut after leaving nearly 1 m length outside the hole for attachment to trunk line of fuse. The connections of detonating fuse of the blast hole with trunk line are shown in fig. 8.21. Where two or more rows of blast holes are to be fired in one round, delay detonators are used. Short delay detonators or milli second detonators give good fragmentation and reduce ground vibrations, the-latter advantage being important where quarries are situated near areas having buildings and costly engine foundations.
The blasted rock pieces in mechanised quarries fly upto 300 m. All the workers within 300 m of the blasting site .should withdraw to safe places. Warning is given by the shotfirer half an hour before the blasting and again 5 minutes before the blasting by an electric siren, or alternatively by a bugle. All the machinery like shovels, drills etc. should be withdrawn to a place where it is not likely to be damaged by blasted rock or should be suitably protected. The heavy blasting in mechanised quarries should take place during rest interval of workers.
The electrical circuit of detonators is tested for continuity by a circuit tester which is essentially a galvanometer. When everything is in order, the cable from detonator leads is connected to an exploder and the explosives are blasted from a protected place.
Pump-truck arrangement of Maharashtra Explosives Ltd., for SMS System
In opencast mining where blasting is on a large scale, the SMS system, is adopted as a standard practice, involving use of specially designed pump-trucks for transport to the blasting site ingredients required for SMS System. The Site-Mixed Slurry System (SMS), basically comprises a mother support plant where an intermediate non-explosive slurry is, initially, prepared for SMS application. This intermediate slurry subsequently, is transferred to a 10-te capacity stainless steel tank mounted on a 18-te capacity, three-axled 'Ashok Leyland' chassis, suitable for off-highway applications.
On rear side of the pump-truck, two small stainless steel tanks of 150-litres capacity each, are mounted which store cross-linker and sensitizer solutions. Each of these tanks is connected to a stainless steel screw pump powered by a hydraulic motor. The intermediate slurry, cross-linker and sensitizer are simultaneously fed onto a mixing hopper at a pre-calibrated rate, wherefrom it is pumped into blast-holes with the help of a progressive cavity stainless steel pump. The pump delivery is connected to a 15m long, static-charge-resistant chemical hose of 50mm diameter. All the control, check, relief and regulating valves for the hydraulic fluid are mounted at the rear end of the chassis in a control console.
For safety consideration, all storage vessels are made of stainless steel. The discharge hose is antistatic. The exhaust pipe is fitted with a flame-arrestor and a fire screen is provided separating the main body of the pump truck and the cabin.
Computer blasting model: The latest technique in blast design is to use computer simulations which take into account rock properties, blast geometry and explosive characteristics. One such computer model is SABREX, marketed by 1CI (India) Ltd. SABREX stands for 'Scientific' Approach to Breaking Rock with Explosives. The essential requirement for running Sabrex simulations is:
1. Five rock properties; Density, Young's Modulus, Poisson's ratio, compressive strength and tensile strength.
2. Explosive characteristics like shock energy, gas energy VOD, detonation pressure, density, etc. A companion programme called CPeX— Commercial Performance of Explosives — calculates these properties for a given composition and density.
3. Blast geometry in terms of hole diameter, bench height, burden, spacing, charge, stemming, delay pattern, etc.
Given these inputs, SABREX predicts fragment size analysis, throw, muck pile profile, damage envelope, fly rock and cost of drilling blasting. The results are displayed in colour graphics and tabulations. The blasting engineer can experiment with adjustments to many factors in a computer safety and achieve results of practical benefit.
1. SABREX accommodates variations of input to all elements of the blast design.
2. SABREX crack pattern is a view of the cracks extending from each bore hole yielding information on fragmentation, delay periods and back break.
3. SABREX gives a picture of back break damage.
4. SABREX calculates fragmentation in terms of the percentage of passing size.
5. SABREX calculates ‘heave’ and builds muck pile profile to assist in subsequent digging operations.
0
Secondary blasting
Secondary blasting is carried out in two ways:—
1. Pop shooting.
2. Plaster shooting.
Pop Shooting. A hole is drilled by jackhammer for charging with explosive and blasting the boulder. Normally a depth of 0.3 to 0.6 m is sufficient for most of the boulder sizes. The explosive, widely used, is special gelatine in conjunction with safety fuse or detonators.
Plaster Shooting. A charge of explosive consisting of either a single primed cartridge or a few cartridges is laid on the surface of the boulder. It is then covered with a shovelful of plastic clay which is pressed into position by hand. It is advantageous to wet the surface of the stone before plastering and the clay should be well pressed down for good contact with stone round the explosive. Special gelatine, or Ajax G, can be used for the blasting. I.E.L. has developed an explosive known as "Plaster Gelatine" for this purpose. It is a high Velocity, high strength, gelatine type explosive suitable for such work. Piaster gelatine is not in the regular products of I.E.L. but can be available on request. I.O.L. also markets a LOX cartridge which is used for contact blasting.
In plaster shooting the explosive charge used is about four times that required for pop shooting.
The object of secondary blasting is to break oversize boulders produced during the first or primary blasting to a size suitable for loading.
Blasting Gallery Technique
This, is a new technique for depillaring of thick coal seams. Blasting gallery technique was adopted at GDK 10 incline of Singareni Collieries Co. Ltd. with collaboration of M/s Charbonnase 'de France. The method involves drivage of level galleries, and depillaring by drilling holes in a fan pattern around the gallery. The full thickness of the seam is extracted by retreating along the galleries, while blasting and loading out of successive slabs.
Due to the use of longer holes, the method requires special explosives and accessories. The explosives charge (of upto3 kg) is distributed in the hole with the help of spacers, and non-incendive detonating fuse is used to ensure reliable initiation of the explosives column.
ICI has developed special explosive - UNIRING - and a non-incendive detonating fuse - RINGCORD - which have passed special tests and have been approved for use with this method.
The details of depillaring by the method of Blasting Gallery Technique are given in the chapter on Thick Seam Working and the reader should refer to it. p- 431
Coyote blasting:
In this system a large quantity of explosive charge, nearly a few hundred tonnes, is packed in the large chambers inside, the underground mine. These chambers are made by driving tunnels, drifts or raises and are called coyote chambers. After packing explosives in the coyote chambers, the connecting tunnels/drifts/ raises are backfilled tightly with the muck excavated when forming the chambers and the charge is normally blasted with the help of detonating cord.
Cast blasting—
Throw blasting or cast blasting is described as the controlled placement of, overburden by drilling and blasting to achieve the optimum overburden removal cost. The technique is suitable for use with large or small draglines and can be adapted for shovel and dragline operations in conjunction with track dozers to place upto 40% of the overburden in final position.
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